1. Field of the Invention
This invention relates to a coding and decoding system for picture data which employs orthogonal transform which is one of high efficiency compression coding methods, and more particularly to a picture data decompression apparatus which decompresses compressed picture data obtained by orthogonal transform, quantization and variable length coding of original picture data at a high speed.
2. Description of the Related Art
A procedure of processing of a conventional picture data decompression apparatus is illustrated in FIG. 12.
Referring to FIG. 12, compressed picture data are first read in at step S121, and then at step S122, a header of the compressed picture data is analyzed to extract information necessary for decompression of the picture data such as a picture size, a number of blocks to be processed and so forth. Then at step S123, variable length codes of the compressed picture data are decoded in units of one block to obtain quantized orthogonal transform coefficients. At subsequent step S124, the quantized orthogonal transform coefficients are dequantized, and then at step S125, data obtained by such dequantization are written for one block into a memory, and the thus written data are read out from the memory. At step S126, the data thus read out are processed by inverse orthogonal transform to obtain picture data for one block.
The operations from the decoding processing of variable length codes at step S123 to the inverse orthogonal transform processing at step S126 are repeated for all of the blocks extracted in the header analysis processing at step S122 until it is determined at step S127 that processing for all of the blocks has been completed. Then at step S128, the picture data for one frame are written into a frame memory, and then the compressed picture data are decompressed. In this instance, in the decoding processing of variable length codes at step S123, compressed code bits are cut out by a plural number and compared with a code table, and when a coincident bit pattern is found out in the code table, coefficient data corresponding in a one-by-one corresponding relationship to the bit train in the code table can be determined. When no coincident bit pattern is found out in the code table, an additional bit is cut out, and the formerly cut out bits with the additional bit are compared with the code table again. After coefficient data are determined, they are multiplied by a quantization value for dequantization to obtain input data for inverse orthogonal transform.
Further, in the data writing and reading out processing at step S125, the data are written into the memory in order indicated by the numbers of the data construction in the block illustrated in FIG. 13, and data to be used for inverse orthogonal transform are read in order in the vertical direction and the horizontal direction of the block.
In the conventional picture data decompression method described above, a compressed code bit train is cut out by a plurality of bits, and the pattern of the thus cut out bits is compared with a code table. Then, if a coincident pattern is not found out in the code table, then the processing of cutting out a further plurality of bits from the compressed code bit train and comparing the thus cut out bits with the code table again is repeated until a coincident pattern is found out in the code table. Consequently, the conventional picture data compression method is disadvantageous in that it requires much processing time.
Further, in order to write data, which remain arranged in order as zigzag-scanned on the coding side, into a memory after dequantization, processing of scanning conversion is required, and the memory is accessed in units of one coefficient. Consequently, the conventional picture data decompression method has disadvantages also in that a very great number of processing steps are required for the microprocessor and also accessing to the memory is complicated, resulting in much processing time.